Communication technologies that link electronic devices are many and varied, servicing communications via both physical media and wirelessly.
Some communication technologies interface a pair of devices, other communication technologies interface small groups of devices, and still other communication technologies interface large groups of devices. Examples of communication technologies that couple small groups of devices include buses within computers, e.g., PCI (peripheral component interface) bus, ISA (industry standard architecture) bus, USB (universal serial bus), and SPI (system packet interface).
One communication technology for coupling relatively small groups of devices or components is the HyperTransport (HT) technology, previously known as the Lightning Data Transport technology (HyperTransport I/O Link Specification “HT Standard”), which is a data packet-based communication protocol for data transfer over buses. There are currently three versions of HT, with current AMD® Opteron® processors adhering to the HT 2.0b specification, for example. Generally, the HT specifications set forth definitions for a high-speed, low-latency protocol that can interface with other buses like AGP, AGP 8x, PCI, PCI-X, SPI, 1394 (Firewire), USB 2.0, Infiniband, or 1 and 10 Gb Ethernet, for example.
HT interconnects provide high-speed data links between coupled components. Most HT enabled components include at least a pair of HT ports so that HT enabled components may be daisy-chained. In an HT chain or fabric, each coupled component may communicate with each other coupled component using appropriate addressing and control. By way of example, HT generally is used to connect AMD® processors to other AMD® processors or bridge devices.
Many server computers employ HT protocol bus architectures to provide connectivity between the server host system processors and peripheral components. Currently, a bus implementing the Hyper-Transport protocol can be used to potentially transfer data at speeds anywhere from 200 MHz to 3.2 Ghz, depending on the Hyper-Transport protocol version and a number of other factors. The particular speed which is utilized is negotiated during system boot time and is limited by the speed capabilities of the connected device or peripheral. Since a server's overall performance can be limited by the bus speed, it is desirable to operate this bus at the maximum possible speed.
The primary challenge in utilizing the maximum or optimal speed achievable over HT buses is establishing very accurate timing for sampling of the data in the center of the data eye. The HT protocol does not provide any provisions for establishing accurate timing during negotiation of the link. As a result, achieving accurate sampling timing is very challenging at frequencies greater than about 400 MHz because of various factors, such as temperature and voltage variations.